In, for example, Tokkai Hei 4-241754 published by the Japanese patent Office in 1992, it is described how the air-fuel ratio in a combustion chamber of an engine is made non-uniform so that the engine may be stably operated under lean burn conditions. In this engine, in a low or medium load region, only air is aspirated into the combustion chamber during the intake stroke, and fuel is injected from a fuel injection valve oriented toward the combustion chamber in the vicinity of a spark plug also situated in the chamber during the latter half of the compression stroke of the chamber. As a result, a combustible fuel-air mixture builds up in the region of the spark plug, and an air layer which contains hardly any fuel is formed around this combustible mixture. In this case, combustion takes place only in the layer of fuel-air mixture in the vicinity of the spark plug. This type of combustion is known as stratified combustion, and in this type of combustion, it is possible to run the engine at an extremely lean air-fuel ratio in the entire combustion chamber.
However, this lean burn engine has a problem when it is equipped with a fuel purge mechanism. A fuel purge mechanism is so constructed that fuel vapor which has evaporated from a fuel tank is first adsorbed by a canister so that it does not escape into the atmosphere, and is recirculated via an intake passage back into the combustion chamber. The purged fuel and air entering the combustion chamber are in a pre-mixed state, and therefore fuel vapor is present also in the intake air in the outer layer around the combustible mixture.
The air-fuel ratio in this outer air layer is leaner than the limit at which combustion is possible, so in an engine in which stratified combustion is performed, the combustion is completed without the combustion flame propagating to the fuel vapor in this outer layer. The result of this is that air containing fuel vapor is discharged into the atmosphere, and hydrocarbons (HC) increase in the exhaust of the engine.
In this regard, Tokkai Hei 5-223017 published by the Japanese patent Office in 1993 discloses how recirculation of fuel vapor into the intake air is stopped and purge of fuel vapor is not performed in the low/medium load region of an engine in which stratified combustion is performed.
Fuel vapor is therefore mixed with intake air only in a high load region of the engine wherein pre-mixing combustion is performed, i.e. in this load region fuel injection timing by the fuel injection valve is advanced such that fuel and air are mixed to form a uniform mixture preceding the combustion.
However, fuel vapor from the fuel tank continues to be adsorbed by the canister. Consequently, when the engine is operated for a long period of time in the low/medium load region wherein purged fuel is not recirculated, the canister becomes saturated with fuel vapor, and part of the fuel adsorbed by the canister may leak into the fuel supply system of the engine or into the atmosphere.